The present invention relates to a novel adhesive composition and a coverlay film prepared therewith for protecting a flexible printed circuit board having excellent adhesiveness and heat resistance to withstand soldering. A coverlay film consists of a heat resistant plastic film or sheet as a substrate and a layer of an adhesive in a semi-cured or solvent-free state on one surface of the substrate.
Along with the trend in recent years in the electronic instruments toward further and further decreased weight, thickness and size still with more and more increased and upgraded performance, the demand for flexible printed circuit boards is rapidly growing with expansion of the application fields in which they are used. The growing use of flexible printed circuit boards is necessarily accompanied by more and more frequent use of coverlay films for protection of flexible printed circuit boards and performance thereof is eagerly desired to be improved to comply with any high-grade applications. Particular problems of recent frequent issues relative to the coverlay films include the adhesiveness to flexible printed circuit boards, high heat resistance to withstand soldering, electric insulation with low moisture absorption, flexibility, weatherability under adverse ambient conditions and the like. Namely, it is desired that a coverlay film should be imparted simultaneously with improved characteristics in all of these properties.
As is mentioned above, a coverlay film is a laminate consisting of a heat resistant plastic film or sheet and a layer of an adhesive in a semi-cured or solvent-free state usually covered with a sheet of surface-release paper or plastic film for temporary protection from inadvertent sticking. Various kinds of adhesives are conventionally used in the prior art for the purpose including blends of NBR and a phenolic resin, NBR and an epoxy-containing phenolic resin, NBR and an epoxy resin, an epoxy resin and a polyester resin and an epoxy resin and acrylic resin as well as acrylic resins as such. These conventional adhesives have their own advantages and disadvantages and none of them can satisfy all of the requirements simultaneously. For example, the adhesives compounded with an NBR are generally poor in the thermal stability and their thermal degradation is accelerated by compounding with an inorganic flame retardant. Epoxy resin-based adhesives in general exhibit relatively low peeling resistance while brominated epoxy resin-based ones as well as polyester-based and acrylic adhesives are poor in the heat resistance. Furthermore, the coverlay films prepared by using the above described conventional adhesives are disadvantageous in respect of the low workability because the layer of the adhesive can be cured at a high temperature of 170.degree. to 180.degree. C. taking as long as 30 to 60 minutes after the coverlay film freed from the sheet of release paper is applied to the surface of a flexible circuit board.